Distributed energy resource units, like solar cells or wind turbines, connected to a common electricity network can provide in this electricity network islands, e.g., parts of the common electricity network, which can be provided with power generation of their own. A three-phase islanding state is a loss-of-mains situation in which all three phases of an electricity network incorporating power generation lose connection with the rest of the common electricity network. It is important to detect a three-phase islanding state because it causes a safety hazard within the electricity network separated from the common electricity network.
In a known method a power supply assembly supplying power in a three-phase electricity network is controlled by a frequency reference signal adapted to deviate the output frequency of the power supply assembly from a grid frequency, wherein a three-phase islanding state is detected if an output frequency of the power supply assembly is outside an allowable value range. The grid frequency represents a frequency of a common electricity network whose part the three-phase electricity network is in normal operating conditions. The frequency reference signal is formed by multiplying a difference between the output frequency of the power supply assembly and the grid frequency with a disturbance constant.
The known methods for three-phase islanding detection increase harmonic distortion in the electricity network and/or detect a three-phase islanding state slowly depending on a magnitude of the disturbance constant.